US4162371A - Method of and means for establishing two-way communication between two stations interconnected by a single signal link - Google Patents

Method of and means for establishing two-way communication between two stations interconnected by a single signal link Download PDF

Info

Publication number
US4162371A
US4162371A US05869171 US86917178A US4162371A US 4162371 A US4162371 A US 4162371A US 05869171 US05869171 US 05869171 US 86917178 A US86917178 A US 86917178A US 4162371 A US4162371 A US 4162371A
Authority
US
Grant status
Grant
Patent type
Prior art keywords
signal
outgoing
stations
means
link
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
US05869171
Inventor
Piero Belforte
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Centro Studi e Laboratori Telecomunicazioni SpA (CSELT)
Original Assignee
Centro Studi e Laboratori Telecomunicazioni SpA (CSELT)
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Grant date

Links

Images

Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04LTRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L5/00Arrangements affording multiple use of the transmission path
    • H04L5/14Two-way operation using the same type of signal, i.e. duplex
    • H04L5/1423Two-way operation using the same type of signal, i.e. duplex for simultaneous baseband signals

Abstract

Two stations of a telecommunication system, interconnected by a single two-way signal link, have respective signal transmitters each working into an impedance network with a first branch connected to the two-way link and a second branch forming a voltage divider. Each station also has a signal receiver energized through a comparator which recovers the incoming signal from a composite signal present at a junction point between the two-way link and the first network branch by subtracting a balancing signal, available at the voltage divider, from this composite signal. The comparator may comprise a differential amplifier or a digital subtractor.

Description

FIELD OF THE INVENTION

My present invention relates to a method of and circuitry for facilitating simultaneous two-way communication between a pair of stations of a telecommunication system interconnected by a bidirectional signal link.

BACKGROUND OF THE INVENTION

Duplexing circuits, designed to enable the simultaneous exchange of incoming and outgoing signals between two interconnecting stations, conventionally comprise separate signal paths (such as transmission lines) facilitating traffic in respective directions. The duplication of the signal path is expensive, especially over longer distances, and also complicates ancillary operations such as the checking of the integrity of the communication channel.

OBJECTS OF THE INVENTION

An important object of my present invention, therefore, is to provide a method of facilitating the simultaneous bidirectional exchange of messages with the use of but a single two-way signal link.

A related object is to provide duplexing circuitry for implementing this method.

SUMMARY OF THE INVENTION

In accordance with my present invention, an outgoing signal generated at either of the two more or less identical intercommunicating stations is split between the signal link and a local circuit, the component of this signal fed to the signal link being thus superimposed upon an incoming signal from the opposite station whereby a composite signal is formed. In order to recover this incoming signal, i.e., to isolate it from the outgoing component, I derive from the local circuit a balancing signal substantially identical with that component whose subtraction from the composite signal yields the incoming signal.

The circuitry at each station designed to carry out this method comprises, in accordance with another aspect of my invention, an impedance network connected to a signal generator which produces the outgoing signal, this network having a first branch connected to the signal link and a second branch provided with step-down means such as a voltage divider. A comparator, such as a differential amplifier, has a first input connected to a junction of the first network branch with the signal link and a second input connected to the step-down means (e.g., to a center tap of the voltage divider) so as to receive on the one hand the composite signal and on the other hand the balancing signal referred to above. A resulting difference signal, appearing in the output of the comparator, is fed to the signal receiver of the station.

Such circuitry can be designed for both analog and digital signals and can be realized in integrated form.

BRIEF DESCRIPTION OF THE DRAWING

The above and other features of the invention will now be described in detail with reference to the accompanying drawing in which:

FIG. 1 is a block diagram showing the overall layout of a telecommunication system embodying my invention;

FIG. 2 is a more detailed circuit diagram of two intercommunicating stations forming part of the system of FIG. 1;

FIG. 3 is a circuit diagram similar to FIG. 2, illustrating a modification; and

FIG. 4 is a circuit diagram of one such station, showing a further modification.

SPECIFIC DESCRIPTION

In FIG. 1 I have shown a telecommunication system with two subsystems A' and A" each comprising a multiplicity of transmit/receive stations RT1 '-RTn ' and RT1 "-RTn ". Stations paired with each other in the two subsystems, indicated by identical subscripts, are interconnected by two-way signal links L1 -Ln. Each station is seen to comprise a transmitting section TM1 '-TMn ', TM1 "-TMn " and a receiving section RC1 '-RCn ', RC1 "-RCn ", the two sections of each station being both connected to the associated signal link via an impedance network N1 '-Nn ', N1 "-Nn ".

FIG. 2 shows details of a pair of such stations RT', RT" interconnected by a singal link L here shown as a coaxial cable; the link could also comprise a two-wire line, for example. The transmitter TM', TM" of each station is symbolically represented by a signal generator producing respective signals +e', +e" (the polarity being, of course, arbitrarily chosen). Each impedance network N', N" has a first branch R', R", i.e., and impedance (here shown as an ohmic resistance) in series with line L with which it forms a junction P', P", and a second branch forming a voltage divider PR', PR" consisting of two identical resistors Rp', Rp". The center tap T', T" of this voltage divider is connected to a subtractive input of a differential amplifier AD', AD" whose additive input is tied to the respective junction point P', P". The differential amplifier, comparing the two signals present in its inputs, works into the receiver RC', RC" of the station.

With line L terminated at both ends by matching impedances R' and R", each transmitter TM', TM" sees that line as an impedance Z in series with a resistance R' or R" of like magnitude; thus, the outgoing signal e'(t), varying as a function of time t, appears with half its magnitude at junction point P'. With a component of signal e"(t) from station RT" coming in over line L, a composite signal E'=[e'(t)+e"(t-tp)]/2 appears at point P' with tp representing the propagation time over line L. In an analogous manner, a composite signal E"=[e"(t)+e'(t-tp)]/2 is concurrently present at point P".

A balancing or compensating signal B' present at tap T' of divider PR' has a magnitude e'(t) which equals that of the component of outgoing signal e'contributing to composite signal E'. Thus, differential amplifier AD' produces the difference E'-B' which, upon amplification by a factor of 2, equals (except for the delay tp) the voltage e" generated by transmitter TM" of station RT". Receiver RC', accordingly, is energized only with the incoming signal.

In a like manner, amplifier AD" subtracts a balancing signal B" from composite signal E" present at point P" to recover the signal e'emitted by station RT'.

In FIG. 3 I have shown a similar system wherein, however, a signal link LL interconnecting the two stations RT' and RT" has a characteristic impedance so low as to constitute practically a short circuit. In this case the network branches R', PR' and R", PR", aside from being equal to one another, need not have any specific magnitude. FIG. 3 also indicates that, in lieu of purely ohmic resistances as shown in FIG. 2, these network impedances could be partly reactive; in particular, I have shown resistors R' and R" shunted by capacitors K' and K", similar capacitors C' and C" (of a combined capacitance equaling that of capacitors K' and K") being connected in parallel with voltage-divider sections Rp' and Rp". In this system the time delay tp will be practically zero.

The foregoing description contemplates an idealized situation with transmitters TM' and TM" having zero internal impedance so as to shunt the incoming signals to ground and with receivers RC', RC" having infinite input impedances; it is also assumed that lines L and LL are practically loss-free. Such an idealized situation will be approximately with many actual transmission systems, especially those operating with binary signals. If, however, line losses become significant, the step-down factor of 1:2 provided by voltage dividers PR" and PR" no longer applies. The various network impedances will then have to be differently dimensioned to take the actual circuit parameters into account, i.e., to provide balancing signals B', B" compensating for the contributions of the outgoing signals e', e" to the composite signals E', E".

In cases where a significant part of the incoming signal (such as e") appears at the input of the step-down circuit (e.g., PR'), that part ought to be filtered out so as not to enter into the balancing signal (e.g., B'). A convenient way of eliminating this part of the incoming signal is by digital filtering. Thus, in the case of analog signals, a conversion into digital signals and subsequent reconversion to analog form will be desirable. FIG. 4 shows such an arrangement wherein station RT' (which of course is also representative of companion station RT") includes a first analog/digital converter ANI, inserted between junction point P' and a positive input of a binary adder S, and a second analog/digital converter ANII, inserted between the output of transmitter TM' and the negative input of binary adder S in series with a digital filter FN. The latter extracts from the output of converter ANII the proper fractional part of digitized signal e' to balance the contribution of this signal to the composite signal appearing in digitized form at the output of converter ANI. The difference signal produced by adder S is amplified in a digital multiplier M to substantially the original level of the incoming signal, as described above with reference to amplifiers AD' and AD". Multiplier M works into a digital/analog converter NA which feeds the receiver RC'.

It will be apparent that the cost of the additional circuitry provided by my invention at each station, even with analog/digital conversion as in FIG. 4, will generally be outweighed by the saving resulting from elimination of an entire signal path. The increased operating convenience represents an additional benefit.

Integrated circuitry for realizing the various network components is well known in the art.

Although the system described above uses signals which are unbalanced with reference to ground, it will be understood that balanced signals could be transmitted and received in a similar manner.

Claims (13)

I claim:
1. A method of facilitating simultaneous signaling in two directions between a pair of stations of a telecommunication system interconnected by an unbalanced transmission line constituting a two-way signal link, comprising the steps of:
generating, at each of said stations, an outgoing signal as a first unbalanced voltage with reference to ground;
splitting said outgoing signal between a conductor of said transmission line and a local circuit;
extracting, at each of said stations, from said conductor a composite signal as a second unbalanced voltage including an incoming signal and a component of said outgoing signal;
deriving from said local circuit a compensating signal substantially identical with said component; and
subtracting said compensating signal from said composite signal, thereby producing a further unbalanced voltage substantially corresponding to said incoming signal.
2. A method as defined in claim 1, comprising the further step of amplifying at each of said stations the isolated incoming signal to substantially the level of the corresponding outgoing signal generated at the opposite station.
3. A method as defined in claim 1 wherein said balancing signal is derived from the portion of said outgoing signal fed to said local circuit by stepping down said portion to a predetermined fraction.
4. In a telecommunication system comprising a pair of stations interconnected by a two-way signal link,
the combination therewith of circuitry at each of said stations facilitating simultaneous exchange of outgoing and incoming signals with the other station, said circuitry comprising:
signal-generating means producing an outgoing signal;
an impedance network connected to said signal-generating means, said network having a first branch connected to said signal link and a second branch provided with step-down means including a binary subtractor;
comparison means with a first input connected to a junction of said first branch with said signal link for obtaining therefrom a composite signal and with a second input connected to said step-down means for obtaining therefrom a balancing signal substantially identical with a component of said outgoing signal fed via said first branch to said signal link; and
signal-receiving means connected to said comparison means for obtaining therefrom an incoming signal produced by subtracting said balancing signal from said composite signal.
5. The combination defined in claim 4 wherein said step-down means comprises a digital filter.
6. The combination defined in claim 5 wherein said outgoing and incoming signals are analog voltages, said impedance network further comprising a first analog/digital converter between said junction and said first input, a second analog/digital converter between said signal-generating means and said digital filter, and a digital/analog converter between said comparison means and said signal-receiving means.
7. The combination defined in claim 4, further comprising a digital multiplier inserted between said binary subtractor and said signal-receiving means.
8. A method of facilitating simultaneous signaling in two directions between a pair of stations of a telecommunication system interconnected by a two-way signal link comprising the steps of:
generating, at each of said stations, a first analog voltage fed in part as an outgoing signal to said signal link;
digitizing another part of said first analog voltage to form a first binary signal;
extracting, at each of said stations, from said signal link a composite signal in the form of a second analog voltage including an incoming signal and a component of said outgoing signal;
digitizing said second analog voltage to form a second binary signal;
stepping down said first binary signal in a digital filter to form a third binary signal balancing a part of said second binary signal corresponding to said component;
subtracting said third binary signal from said second binary signal to form a resulting binary signal; and
converting said resulting binary signal to analog form, thereby substantially reconstituting said incoming signal.
9. A method as defined in claim 8, comprising the further step of digitally multiplying said resulting binary signal before converting same to analog form.
10. In a telecommunication system comprising a pair of stations interconnected by a conductor forming a two-way signal link,
the combination therewith of circuitry at each of said stations facilitating simultaneous exchange of outgoing and incoming signals with the other station, said circuitry comprising:
signal-generating means producing an outgoing signal unbalanced with reference to ground;
an impedance network connected to said signal-generating means, said network having a first branch connected to said conductor and a second branch provided with step-down means;
comparison means with a first input connected to a junction of said first branch with said conductor for obtaining therefrom a composite signal and with a second input connected to said step-down means for obtaining therefrom a compensating signal substantially identical with a component of said outgoing signal fed via said first branch to said conductor; and
signal-receiving means connected to said comparison means for obtaining therefrom an incoming signal produced by subtracting said compensating signal from said composite signal.
11. The combination defined in claim 10 wherein said comparison means comprises a differential amplifier.
12. The combination defined in claim 10 wherein said signal link is a line with a significant characteristic impedance, said first branch comprising a series impedance substantially equaling said characteristic impedance, said step-down means being a voltage divider with an overall impedance substantially equaling said characteristic impedance.
13. The combination defined in claim 12 wherein said second input is connected to a center tap of said voltage divider.
US05869171 1977-01-14 1978-01-13 Method of and means for establishing two-way communication between two stations interconnected by a single signal link Expired - Lifetime US4162371A (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
IT67083A/77 1977-01-14
IT6708377A IT1116705B (en) 1977-01-14 1977-01-14 Improvements to systems of rice transmission of signals

Publications (1)

Publication Number Publication Date
US4162371A true US4162371A (en) 1979-07-24

Family

ID=11299427

Family Applications (1)

Application Number Title Priority Date Filing Date
US05869171 Expired - Lifetime US4162371A (en) 1977-01-14 1978-01-13 Method of and means for establishing two-way communication between two stations interconnected by a single signal link

Country Status (4)

Country Link
US (1) US4162371A (en)
DE (1) DE2801469C2 (en)
FR (1) FR2377732B1 (en)
GB (1) GB1592831A (en)

Cited By (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0026931A1 (en) * 1979-10-04 1981-04-15 CSELT Centro Studi e Laboratori Telecomunicazioni S.p.A. Transceiver for full duplex transmission of digital signals on a single line
US4272843A (en) * 1978-06-13 1981-06-09 Licentia Patent-Verwaltungs-G.M.B.H System for transmitting service communications
EP0186131A2 (en) * 1984-12-28 1986-07-02 Gte Laboratories Incorporated Digital switching system with two-directional addressing rams
EP0186142A2 (en) * 1984-12-28 1986-07-02 Gte Laboratories Incorporated Two wire bidirectional digital transmission system
GB2204765A (en) * 1987-05-09 1988-11-16 Gewerk Eisenhuette Westfalia Data transmission/reception systems for electro-hydraulic control systems
EP0405743A2 (en) * 1989-06-29 1991-01-02 Digital Equipment Corporation Bidirectional transceiver for high speed data system
US5216667A (en) * 1991-05-24 1993-06-01 International Business Machines Corporation Simultaneous bidirectional transceiver
US5541535A (en) * 1994-12-16 1996-07-30 International Business Machines Corporation CMOS simultaneous transmission bidirectional driver/receiver
US5587824A (en) * 1991-07-26 1996-12-24 Cybex Computer Products Corporation Open collector communications link
US5666354A (en) * 1995-12-20 1997-09-09 International Business Machines Corporation CMOS bi-directional differential link
EP0686920A3 (en) * 1994-06-06 1997-10-29 Jeong Deog Kyoon High speed serial link for fully duplexed data communication
US6522701B1 (en) 1998-10-07 2003-02-18 Conexant Systems, Inc. Method and apparatus for extracting received digital data from a full-duplex point-to-point signaling channel using sampled-data techniques
US20050176386A1 (en) * 2004-02-10 2005-08-11 Brodhead Colin D. System and method for transmitting data via wave reflection
US20060140284A1 (en) * 2004-12-28 2006-06-29 Arthur Sheiman Single conductor bidirectional communication link
JP2012138680A (en) * 2010-12-24 2012-07-19 Sony Corp Full duplex transmission circuit and electronic apparatus
US8305099B2 (en) * 2010-08-31 2012-11-06 Nxp B.V. High speed full duplex test interface

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2870288B2 (en) * 1992-03-17 1999-03-17 株式会社日立製作所 Bidirectional signal transmission circuit

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3519743A (en) * 1966-07-14 1970-07-07 Int Standard Electric Corp Circuit arrangement for simultaneous signalling in both transmission directions between two terminal stations in telecommunication systems
US3862364A (en) * 1971-12-15 1975-01-21 Hitachi Ltd Transmitting-and-receiving apparatus for performing data transmission through common bus
US4012590A (en) * 1974-10-31 1977-03-15 Siemens Aktiengesellschaft Circuit arrangement for two-wire full duplex data transmission

Family Cites Families (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2886812A (en) * 1959-05-12 morgan
DE2019395B2 (en) * 1970-04-22 1980-06-12 Telefonbau Und Normalzeit Gmbh, 6000 Frankfurt Coupling circuit for telephone set - has two differential amplifiers cross-coupled between single bidirectional transmission line, microphone and loudspeaker
JPS5226972B2 (en) * 1973-07-28 1977-07-18

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3519743A (en) * 1966-07-14 1970-07-07 Int Standard Electric Corp Circuit arrangement for simultaneous signalling in both transmission directions between two terminal stations in telecommunication systems
US3862364A (en) * 1971-12-15 1975-01-21 Hitachi Ltd Transmitting-and-receiving apparatus for performing data transmission through common bus
US4012590A (en) * 1974-10-31 1977-03-15 Siemens Aktiengesellschaft Circuit arrangement for two-wire full duplex data transmission

Cited By (30)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4272843A (en) * 1978-06-13 1981-06-09 Licentia Patent-Verwaltungs-G.M.B.H System for transmitting service communications
US4393494A (en) * 1979-10-04 1983-07-12 Cselt Centro Studi E Laboratori Telecomunicazioni S.P.A. Transceiver for full-duplex transmission of digital signals over a common line
EP0026931A1 (en) * 1979-10-04 1981-04-15 CSELT Centro Studi e Laboratori Telecomunicazioni S.p.A. Transceiver for full duplex transmission of digital signals on a single line
EP0186142A3 (en) * 1984-12-28 1988-03-30 Gte Laboratories Incorporated Two wire bidirectional digital transmission system
EP0186131A2 (en) * 1984-12-28 1986-07-02 Gte Laboratories Incorporated Digital switching system with two-directional addressing rams
US4638473A (en) * 1984-12-28 1987-01-20 Gte Laboratories Incorporated Two wire bidirectional digital transmission system
EP0186131A3 (en) * 1984-12-28 1988-04-06 Gte Laboratories Incorporated Digital switching system with two-directional addressing rams
EP0186142A2 (en) * 1984-12-28 1986-07-02 Gte Laboratories Incorporated Two wire bidirectional digital transmission system
GB2204765B (en) * 1987-05-09 1991-04-03 Gewerk Eisenhuette Westfalia Data transmission/reception systems for electro-hydraulic control systems
GB2204765A (en) * 1987-05-09 1988-11-16 Gewerk Eisenhuette Westfalia Data transmission/reception systems for electro-hydraulic control systems
US4899332A (en) * 1987-05-09 1990-02-06 Gewerkschaft Eisenhutte Westfalia Gmbh Data transmission/reception systems for electro-hydraulic control systems
US5579336A (en) * 1989-06-29 1996-11-26 Digital Equipment Corporation Bidirectional transreceiver for high speed data system
EP0405743A3 (en) * 1989-06-29 1992-06-03 Digital Equipment Corporation Bidirectional transceiver for high speed data system
EP0405743A2 (en) * 1989-06-29 1991-01-02 Digital Equipment Corporation Bidirectional transceiver for high speed data system
US5253249A (en) * 1989-06-29 1993-10-12 Digital Equipment Corporation Bidirectional transceiver for high speed data system
US5216667A (en) * 1991-05-24 1993-06-01 International Business Machines Corporation Simultaneous bidirectional transceiver
US5587824A (en) * 1991-07-26 1996-12-24 Cybex Computer Products Corporation Open collector communications link
US5802103A (en) * 1994-06-06 1998-09-01 Sun Microsystems, Inc. High speed serial link for fully duplexed data communication
US6211714B1 (en) 1994-06-06 2001-04-03 Sun Microsystems, Inc. System for Distributing Clocks
EP0686920A3 (en) * 1994-06-06 1997-10-29 Jeong Deog Kyoon High speed serial link for fully duplexed data communication
US5541535A (en) * 1994-12-16 1996-07-30 International Business Machines Corporation CMOS simultaneous transmission bidirectional driver/receiver
US5666354A (en) * 1995-12-20 1997-09-09 International Business Machines Corporation CMOS bi-directional differential link
US6522701B1 (en) 1998-10-07 2003-02-18 Conexant Systems, Inc. Method and apparatus for extracting received digital data from a full-duplex point-to-point signaling channel using sampled-data techniques
US20050176386A1 (en) * 2004-02-10 2005-08-11 Brodhead Colin D. System and method for transmitting data via wave reflection
US7606537B2 (en) 2004-02-10 2009-10-20 Colin Dugald Brodhead System and method for transmitting data via wave reflection
US20060140284A1 (en) * 2004-12-28 2006-06-29 Arthur Sheiman Single conductor bidirectional communication link
US7792196B2 (en) * 2004-12-28 2010-09-07 Intel Corporation Single conductor bidirectional communication link
US20100232485A1 (en) * 2004-12-28 2010-09-16 Arthur Sheiman Single conductor bidirectional communication link
US8305099B2 (en) * 2010-08-31 2012-11-06 Nxp B.V. High speed full duplex test interface
JP2012138680A (en) * 2010-12-24 2012-07-19 Sony Corp Full duplex transmission circuit and electronic apparatus

Also Published As

Publication number Publication date Type
FR2377732A1 (en) 1978-08-11 application
GB1592831A (en) 1981-07-08 application
DE2801469C2 (en) 1984-06-28 grant
DE2801469A1 (en) 1978-07-20 application
FR2377732B1 (en) 1980-04-04 grant

Similar Documents

Publication Publication Date Title
US3499999A (en) Closed loop adaptive echo canceller using generalized filter networks
US4845746A (en) Echo canceller with relative feedback control
US5185591A (en) Power distribution line communication system for and method of reducing effects of signal cancellation
US5007050A (en) Bidirectional digital serial interface for communication digital signals including digitized audio between microprocessor-based control and transceiver units of two-way radio communications equipment
US5301208A (en) Transformer bus coupler
US4507793A (en) Digital signal transmission system
US4918725A (en) Terminal impedance setting circuit for a two-wire to four-wire converting circuit
US4475215A (en) Pulse interference cancelling system for spread spectrum signals utilizing active coherent detection
US4751730A (en) Process and system for improving echo cancellation within a transmission network
US5034978A (en) Universal channel unit switchable between two wire and four wire configurations
US4053717A (en) Cordless telephone
US3530260A (en) Transistor hybrid circuit
US6064422A (en) Telecommunication system for broadcast quality video transmission
US3934099A (en) Bias, feedback and network arrangements for hybrid circuits
US4192978A (en) Operational amplifier hybrid system
US4493092A (en) Interface circuit for digital signal transmission system
US4394624A (en) Channelized feed-forward system
US5548838A (en) Interference cancellation system employing a polar vector modulator
US4393494A (en) Transceiver for full-duplex transmission of digital signals over a common line
US5952914A (en) Power line communication systems
US4605902A (en) Hybrid junction signal combiner
US5153875A (en) Adaptive balancing network
US4139731A (en) Telephone conference system with active analog conference
US4567331A (en) Electronic hybrid having synthesized impedance circuitry
US4024359A (en) Continuity-checking network for telecommunication system